Lasers are widely used in dermatological applications such as hair removal, removal of pigmented lesions, tattoos, vascular lesions, wrinkles, acne, and skin tightening. These and similar applications are effected by heating only a small structure in the skin to a temperature above that which initiates a healing process that removes or replaces the small structure. These laser treatments are typically based on selective targeting of a chromophore in the skin by an appropriate choice of wavelength and pulse duration of the laser light. For example, the blood vessels in vascular lesions are irradiated with a color of light, usually yellow, that is well absorbed by the blood. The absorbed light heats the blood and the blood then heats the vessel walls. Lasers that are able to emit pulses of high peak power work well because the heat builds up in the vessel walls faster than it can be conducted through the wall to the surrounding skin tissue. Most of the heat is confined to the vessel and so it is damaged while most of the surrounding tissue is spared. Treatment pulse durations of several milliseconds are often most effective but effective pulse durations can range from a hundred of microseconds to hundreds of milliseconds. Some lasers can be pumped by flashlamps to get the large pulse energies required for creating the desired thermal profile in the skin.
Several lasers are well suited for use in medical applications. Some high power, pulsed lasers are the preferred instruments for treating certain dermatological conditions. Many of these lasers can be pumped with flashlamps. Flashlamps offer an economical means of delivering several megawatts of pump power in short duration pulses. Examples of flashlamp pumped medical lasers are dye lasers and solid state lasers such as Alexandrite, Ruby and Nd:YAG. Some flashlamp pumped lasers are most efficient when the pulse duration of the laser is shorter than the desired treatment pulse duration for heating the tissue target. In these cases, the laser can be made to emit a long train of short sub-pulses that are spaced in time to produce an effective pulse duration that equals the desired treatment pulse duration.
Some driver circuits for flashlamps use a series of energy storage capacitors that can be discharged sequentially through the flashlamp to generate a train of laser sub-pulses. Each capacitor can be individually charged to different voltages so that the magnitude of the flashlamp current in the sequentially produced flashlamp sub-pulses can be made to increase over that of the previous sub-pulse. The maximum number of sub-pulses in a driving configuration, however, using a series of energy storage capacitors is limited to the number of capacitors. Another disadvantage is that, in practice, the energy delivered in each laser sub-pulse needs to be adjusted individually for the desired output energy. This system becomes complex and difficult to calibrate for a large numbers of sub-pulses.
In other driver circuits, an IGBT is used to connect an energy storage capacitor to the flashlamp. The amplitude of the flashlamp current is determined by the capacitor voltage and the impedance of the flashlamp. The problem with this circuit is that the capacitor voltage drops during the pulse. Therefore, the flashlamp current and hence the peak power of the resultant laser pulse both decrease during the pulse. When used in a multiple sub-pulse mode, each laser sub-pulse has a lower peak power than the previous sub-pulse. At some point in time the laser gain can fall below threshold and lasing ceases. This is a problem for lasers, such as pulsed dye lasers, that suffer a drop in the laser efficiency during a single pulse and in each subsequent sub-pulses due to thermal and other degradations in the lasing medium.